The ecological concept of human carrying capacity is necessarily complicated because human beings are the 'ultimate ecosystem engineers' who moderate the environment for their benefit. For at least the last few hundred years, human ingenuity, access to massive stocks of fossil fuels, and technological development have driven facilitation whereby increasing human abundance has promoted higher population growth rates. However, this positive relationship broke down during the 1950s, and by 1962, the global human population entered a phase where the growth rate consistently declined as population increased. The onset of this negative phase occurred 8 years before a global biocapacity deficit began in 1970. The onset of the negative phase also varies regionally, with the lowest-income and highest fertility regions entering this phase later than higher-income regions. A Ricker logistic model fitted to the negative phase predicts that the global population could reach 11.7-12.4 billion people between 2067 and 2076. The same model fitted to the facilitation phase predicts a maximum population of 2.5 billion people that Earth might be able to maintain. The negative phase also correlates strongly with the trend in global temperature anomaly, ecological footprint, and total emissions, with more of their variation explained by increasing population size rather than increasing per-capita consumption. The Earth cannot sustain the future human population, or even today's, without a major overhaul of socio-cultural practices for using land, water, energy, biodiversity, and other resources.

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Mass extinctions during the past 500 million y rapidly removed branches from the phylogenetic tree of life and required millions of years for evolution to generate functional replacements for the extinct (EX) organisms. Here we show, by examining 5,400 vertebrate genera (excluding fishes) comprising 34,600 species, that 73 genera became EX since 1500 AD. Beyond any doubt, the human-driven sixth mass extinction is more severe than previously assessed and is rapidly accelerating. The current generic extinction rates are 35 times higher than expected background rates prevailing in the last million years under the absence of human impacts. The genera lost in the last five centuries would have taken some 18,000 y to vanish in the absence of human beings. Current generic extinction rates will likely greatly accelerate in the next few decades due to drivers accompanying the growth and consumption of the human enterprise such as habitat destruction, illegal trade, and climate disruption. If all now-endangered genera were to vanish by 2,100, extinction rates would be 354 (average) or 511 (for mammals) times higher than background rates, meaning that genera lost in three centuries would have taken 106,000 and 153,000 y to become EX in the absence of humans. Such mutilation of the tree of life and the resulting loss of ecosystem services provided by biodiversity to humanity is a serious threat to the stability of civilization. Immediate political, economic, and social efforts of an unprecedented scale are essential if we are to prevent these extinctions and their societal impacts.

In spite of a slight slackening of the worlds population growth rate the worlds population is still projected to reach a peak of from 9-12 billion by the turn of the century; this would put enormous strain on the Earths finite resources and ultimately lead to catastrophic mortality rates. Forecasts on food production show that population growth rates in some regions particularly South Asia and tropical Africa are outstripping rate of increases in food supply; developing areas are projected to resort to very large increases in food importation in order to feed the hungry population. Another problem is the rapid deterioration of the ecological system. Many species which are of potential value to man (in terms of potential sources of food fiber drug chemicals) are rapidly being exterminated by man in the name of development projects. Other areas of concern include the need to develop and install new energy technologies to replace conventional ones; development of more efficient transportation and communication systems; improving facilities for production of consumer goods; and building more housing and commercial facilities for the growing population.

How human modification of native habitats changes the feeding patterns and nutritional ecology of tropical birds is critical to conserving avian biodiversity, but tropical bird diets are laborious to investigate using the traditional methods of diet analysis. Stable isotope analysis provides a cost-effective and efficient proxy to identify general foraging patterns, especially when dietary shifts spanning multiple trophic levels have occurred due to ecosystem disturbance or transformation. To characterize the diets of forest bird species that persist in tropical agricultural countryside, we compared feather carbon (δ 13 C) and nitrogen (δ 15 N) isotope values of four species caught and radio-tracked in a 270 hectare forest reserve, smaller forest remnants (including mature forest, secondary forest, and riparian strips), and coffee plantations in mid-elevation ( ca. 800–1,400 m) southern Costa Rica. Bird habitat choice had a significant effect on diet composition as revealed by δ 13 C and δ 15 N values. Three of the four species studied showed evidence of significantly reduced consumption of invertebrates in coffee plantations, with the isotope values of two species ( Tangara icterocephala and Mionectes oleaginous ) indicating, by comparison, nearly a doubling of invertebrate consumption in forest remnants. Our results suggest that coffee plantations are deficient in invertebrates preferred by forest generalist birds that forage in both native forest remnants and coffee plantations. In this region, typical of mountainous American tropics, small forest remnants and a larger forest reserve provide critical dietary resources for native forest birds that utilize the agricultural countryside.

Humanity has triggered the sixth mass extinction episode since the beginning of the Phanerozoic. The complexity of this extinction crisis is centred on the intersection of two complex adaptive systems: human culture and ecosystem functioning, although the significance of this intersection is not properly appreciated. Human beings are part of biodiversity and elements in a global ecosystem. Civilization, and perhaps even the fate of our species, is utterly dependent on that ecosystem's proper functioning, which society is increasingly degrading. The crisis seems rooted in three factors. First, relatively few people globally are aware of its existence. Second, most people who are, and even many scientists, assume incorrectly that the problem is primarily one of the disappearance of species, when it is the existential threat of myriad population extinctions. Third, while concerned scientists know there are many individual and collective steps that must be taken to slow population extinction rates, some are not willing to advocate the one fundamental, necessary, ‘simple’ cure, that is, reducing the scale of the human enterprise. We argue that compassionate shrinkage of the human population by further encouraging lower birth rates while reducing both inequity and aggregate wasteful consumption—that is, an end to growthmania—will be required.This article is part of the theme issue ‘Ecological complexity and the biosphere: the next 30 years’.

Humanity must commit to transformative change on all levels in order to address the climate emergency and biodiversity collapse. In particular, stabilizing and ultimately reducing the human population size is necessary to ensure the long-term wellbeing of our species and other life on Earth. We show how this transition can be accomplished in an equitable framework that promotes human rights. Specifically, we issue a global appeal for women and men to have at most one child and call for policy-makers to implement population policies that improve education for girls and young women and ensure the availability of high-quality family-planning services.